High intensity radiation source



Oct. 18, 1966 L. s. FROST ET AL HIGH INTENSITY RADIATION SOURCE 2 Sheets-Sheet 1 Filed Feb. 28, 1963 2-2252 wz oou INVENTORS Leshe 8. Frost and Howard C. Ludwig.

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Oct. 18, 1966 s FROST ET AL HIGH INTENSITY RADIATION SOURCE 2 Sheets-Sheet 2 Filed Feb. 28, 1963 United States Patent 3,280,360 HIGH INTENSITY RADIATION SOURCE Leslie S. Frost, Penn Hills Township, Allegheny County,

and Howard C. Ludwig, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a

corporation of Pennsylvania Filed Feb. 28, 1963, Ser. No. 261,756 8 Claims. (Cl. 313-231) This invention relates to high intensity light sources and, more particularly, to high intensity light sources which can be operated with a very high power input to generate radiations in a very efiicient manner.

Arc dis-charge devices provide the most intense, artificially generated sources of radiation. If such discharge sources are operated with a very high power input, however, the electrodes tend to overheat. This causes some of the material comprising the electrodes to vaporize and contaminate the envelope of the discharge source. To overcome these problems, it is known to remove some of the discharge plasma from the envelope, as the device operates, and this acts to remove the vaporized electrode material from the discharge source. It is also known to provide an auxiliary cooling means for the electrodes of an arc discharge source, in order to minimize as much as possible the tendency for the electrodes to overheat. Such discharge sources are generally described in US. Patent No. 3,054,921, dated September 18, 1962, and US. Patent No. 3,064,153, dated November 13, 1962. Even these modified discharge sources, however, cannot be operated for prolonged periods of time with very high power inputs without either vaporizing electrode material onto the envelopes or sacrificing efficiency in the generation of radiations.

It is the general object of this invention to provide a high-intensity discharge source which can be operated at very high powers to efficiently convert electrical energy into radiations.

It is another object of this invention to provide a highintensity radiation source which can be operated for long periods of time and which focuses the generated radiations.

It is a further object to provide a high-intensity radiation source having a specially formed anode which provides increased efficiency of operation and minimizes vaporization of electrode material. It is an additional object to provide construction details for a high-intensity radiation source. I

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing an envelope which preferably has a radiation-reflecting internal surface with a focal point of this reflecting surface positioned within the envelope. A portion of the envelope is radiation transmitting and radiations which are generated proximate to the focal point are ultimately directed toward the radiation-transmitting portion of the envelope. A cooled anode and a cooled cathode are operatively positioned about the focal point and are energizable to sustain an intense arc discharge therebetween. An aperture is provided through the anode and extends through the envelope. During operation of the device, the dischargesustaining gas is caused to pass toward the anodic aperture provided through the envelope. A portion of this gas passes through the anodic aperture as discharge plasma, carrying with it any vaporized electrode material. The source is so designed that the gas input to the device, the electrical power input to the device, and the size of the aperture through the envelope cause the gas-operating pressure within the envelope to be at least atmospheres, and the total energy which is dissipated as plasma exiting 3,289,350 Patented Oct. 18, 1966 through the anodic aperture is less than the total energy which is directed as radiations toward the radiation-transmitting envelope portion. Preferably, the anode has a particular configuration which increases the life and efficiency of the device and the discharge-sustaining gas is introducedinto the device in a vortical path in order to provide stability for the formed are.

For a better understanding of the invention, reference should be had to the accompanying drawings, wherein:

FIGURE 1 is a vertical longitudinal sectional view of a discharge source fabricated in accordance with the present invention;

FIG. 2 is a cross-sectional view taken on the line II-II in FIG. 1 in the direction of the arrow;

FIG. 3 is a fragmentary, enlarged longitudinal sectional view of the anode and the cathode, as shown in FIG. 1;

FIG. 4 is an end view of the anode taken on the line IVIV in FIG. 3, in the direction of the arrows;

FIG. 5 is a cross-sectional view taken on the line VV in FIG. 3, in the direction of the arrows;

FIG. 6 is a vertical cross-sectional view taken on the line VIVI in FIG. 1, in the direction of the arrows;

FIG. 7 is a horizontal sectional view taken on the line VII-VII in FIG. 1, in the direction of the arrows;

FIG. 8 is a view of an alternative embodiment of an envelope window; and

FIG. 9 is a sectional view taken on the line VIIIVIII in FIG 1, in the direction of the arrows, illustrating an alternative embodiment of the gas manifold which is shown in FIG. 1.

With specific reference to the form of the invention illustrated in the drawings, the device 10 as shown in FIG. 1 comprises an envelope 12 of predetermined dimensions and configuration which can vary with the power input into the device and the intended use therefor. In the device embodiment as shown in FIG. 1, the main body portion 14 of envelope 12 is formed of stainless steel with a cooling tube 16 encircling the exterior envelope surface. The tube 16 is adapted to carry a flow of cooling water and, during operation of the device, the water is continuously pumped through the tube 16. v

The interior surface of the envelope body portion 14 is formed asa radiation-reflecting ellipse 18 having a first focal point 20 positioned within the envelope and a second focal point 22 positioned exterior to the envelope. Afiixed to the envelope body portion 14 is a stainless steel face member 24 which carries a thick, radiationtransmitting quartz window 26 so positioned with respect to the focal point 20 that substantially all radiations which are generated proximate to this focal point 20 are directed toward the window 26.

Referring to FIG. 2, a series of gas-inlet apertures 28 are positioned to open into the envelope 12 around the quartz window 26 and discharge-sustaining gas such as argon is directed through an inlet tube 30, into a manifold 32, and through the apertures 28 to swirl in a vortical path with respect to the axis of the elliptical reflector 18. The axial motion of the gas passing over the elliptical reflector 18 provides a gas barrier between any vaporized electrode material and the reflector 18 and, in addition, the vortical motion of the gas assists in providing stability for the formed arc.

The anode and cathode are shown in greater detail in FIGS. 3, 4, and 5. The cathode 34 is formed of tungsten and the end portion thereof 6 has the configuration of a truncated cone. To help dissipate the heat which is generated when the source 10 operates, a cooling medium such as water is pumped into the water inlet 38, as shown in FIG. 1, and is circulated into the body of the cathode 3 through the inlet tube 40. The heated water is returned through the outlet tube 42.

The anode 44 is formed of tungsten, copper, or other suitable material having a high thermal conductivity and a high boiling point. The anode 44 is provided with auxiliary cooling means, such as circulating water, in order to help dissipate the heat which is generated. The main body portion of the anode 44 is generally cylindrical terminating in a truncated cone, the end section 46 of which is generally flattened and oriented substantially perpendicular to the cathode. Such an anode construction has been found very desirable to help minimize any tendency for melting of the tip portion of the anode, since the arc is spread over a substantial area of the anode.

This generally flattened anode end portion 46 desirably has an area which is substantially greater than the cross-sectional area of the arc discharge which is adapted to be sustained between the anode and the cathode, in order to minimize formation of anode hot spots. In the embodiment as shown, the generally flattened end portion 46 of anode 44 has a slightly concave configuration. Such a small variance from a completely flat conconfiguration is not detrimental to performance and for some specific embodiments, such a slightly concave configuration can be of some benefit.

An aperture 48 is provided along the axis of the anode 44 and also extends through the envelope 12. The crosssectional area of this aperture 48, as it opens into the generally flattened anode end portion 46, is substantially less than the cross-sectional area of the arc discharge which is adapted to be sustained between the anode 44 and the cathode 34. This is to prevent any appreciable portion of the formed discharge plasma from entering into the aperture 48, which would result in some melting of the anode and, in addition, would represent a great sacrifice in utilization of generated radiations.

To cool the anode 44 during operation, cooling water is introduced to the anode through the inlet tube 50 and is circulated through this tube to the body portion of the anode. The cooling water return is through a concentric return tube 52 and the heated water is discharged at the outlet line 54. In addition to cooling the end portion of the anode, the cooling water also effects a cooling of ,the relatively small amount of plasma which passes the aperture 48. To facilitate plasma cooling and proper operation of the device 10, the aperture 48 enlarges after it passes through the tip portion of the anode, so that the extremely hot plasma is cooled in rapid fashion.

For most eflicient operation, it has been found that the cathode 34 desirably should be adjustable in a direction along the axis of the elliptical reflector 18. Depending upon the conditions of operation, the hottest portion of the formed are may vary slightly with respect to its positioning relative to the cathode 34. To facilitate placing the hottest portion of the are so that it substantially coincides with the focus 20, thecathode 34 is movable a small amount in a direction along the axis of the elliptical reflector 18. This axial movement is accomplished by means of a wheel and screw arrangement 56 which causes the cathode supporting assembly 58 to move to the right or left, as viewed in FIG. 1.

To facilitate starting the device 10,. it is desirable to move the anode 44 into close proximity to the cathode 34 in order to ionize the discharge-sustaining gas. Movement of the anode 44 with respect to the cathode is accomplished by rotating the anode positioning wheel 60. This causes the anode supporting assembly 62 to move axially with respect to the positioning wheel 60, which wheel is held against reciprocal movement by projecting lip 64 which is rigidly aflixed to the reflector body portion 14. As shown in FIG. 6, a keyway 66 prevents the body of the anode from rotating as it is reciprocated. Both the anode and cathode assemblies 62 and 58 are suitably sealed against egress of discharge-sustaining gas from the envelope 12 by suitably placed ring and packing seals. The energizing potential is applied across the cathode and anode assemblies 58 and 62.

As a specific example for practicing the present invention, the power input to the radiation source is 9 kilowatts, and the volume enclosed by the envelope 12 is 4000 cc. Under such conditions of operation, argon is intro duced into the envelope through apertures 23 at a rate of 3.5 cubic feet per minute and at a pressure of 210 p.s.i.a. The operating pressure within the envelope 12 is 210 p.s.i.a. and the cross-sectional area of the aperture 48 provided through the anode end portion 46 is 0.013 sq. cm. The area of the generally flattened anode and portion 46 is 0.8 sq. cm. Under such conditions of operation, the arc is approximately 0.6 cm. long and has an average. cross-sectional area of approximately 0.2 sq. cm. The generation of radiations is extremely efiicient since the amount of energy which is lost as plasma exiting through the aperture 48 is relatively minor. In addition, the swirling discharge-sustaining gas keeps the reflector 18 clean and also tends to stabilize the arc, which in turn enhances the efficiency of operation, particularly where the arc is positioned proximate to a focal point of the reflector. For the foregoing operating conditions as specified, 34 percent of the radiations are concentrated at the exterior focal point 22 and only about 2 percent of the total energy introduced into the lamp is lost as plasma exiting through the aperture 48. The remaining energy input is dissipated as heat through the stainless steel portion of the envelope 12 and removed by the cooling media for the cathode and anode. Of course the intensity of the radiations which are concentrated at the focal point 22 is very great and when the source 10 is operated, a piece of asbestos placed at this focal point 22 will vaporize almost immediately.

Modified sizes of the present device can be operated with greatly increased power input, such as up to 50 kilowatts. For best operation of any such devices, when the power input is from 5 to 50 kilowatts, the gas flow through the aperture 48 desirably should be from 2 to 10 cubic feet per minute.

The discharge source such as described hereinbefore has utility for simulating the radiations which are encountered during re-entry of a capsule or missile. In addition, this source can be of use in conjunction with outdoor movie screen projections, image furnaces, solar simulation searchlights, continuous laser pumping, or other similar applications where a very intense, focused light source is desired.

A possible alternative embodiment for the quartz window construction is shown in FIG. 8 wherein the radiation-transmitting quartz window 68 is formed as a lens which moves the focal point 22 further away from the device 10.

Also shown in FIG. 8 and further illustrated in FIG. 9 is a modified manifold system for introducing dischargesupporting gas into the device 10. In this modified systern, the discharge-sustaining gas is introduced into the manifold 70 and is then forced through the gas-diffusing apertures 72 which are angularly disposed with respect to the axis of the reflector 18, in order to impart a vortical motion to the gas.

As another possible alternative embodiment, the envelope 12 can be modified to eliminate the reflecting portion, with the radiation-transmitting quartz portion circumferentially extending about the anode and cathode. This will provide an omnidirectional light source of high intensity, such as might be used as a navigation aid. As yet another alternative design, the radiation-reflecting portion of the envelope 12 can have a paraboloidal configuration, similar to a sealed-beam lamp as used with automobiles.

While it is preferred to effect starting by moving the anode proximate to the cathode, starting can also be effected by the use of an auxiliary, high-frequency potential to ionize the discharge-sustaining gas.

Discharge-sustaining gases other than argon can be used with modified spectral effects, depending on the gas used. Examples of such other gases are the other rare gases, or mixtures of rare gases. Particularly if the more expensive rare gases are used, it may be desirable to recirculate the gas.

It will be recognized that the objects of the invention have been achieved by providing a high-intensity radiation source which operates with good efficiency. The radiations which are generated by this source can be concentrated at a point exterior to the source, or otherwise focused. In addition, there has been provided an im proved anode construction for such a device, which anode construction minimizes vaporization of the material comprising the anode and also improves efiiciency.

While a best embodiment of the invention has been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

We claim as our invention:

1. A radiation source comprising:

(a) envelope means of predetermined dimensions and configuration, a portion of said envelope means being radiation transmitting, another portion of said envelope means being radiation reflecting and having a focal point positioned Within said envelope means, and the relative positioning of the radiationreflecting envelope portion and the radiation-transmitting envelope portion being such that substantially all radiations generated proximate to said focal point are ultimately directed toward the radiationtransmitting envelope portion;

(b) a cooled anode and a cooled cathode operatively positioned within said envelope means and about said focal point and adapted to be energized with a predetermined power input to sustain an intense arc discharge therebetween and proximate to said focal point;

(c) an aperture of predetermined dimensions provided through said anode and extending through said envelope means and operable to conduct plasma therethrough;

(d) gas-source means terminating in gas-inlet means extending into said envelope means at a location which is positioned about said radiation-transmitting envelope portion and operable, during operation of said source, to pass a selected discharge-sustaining gas at a predetermined pressure into said envelope means to create a gas stream moving toward the anodic aperture provided through said envelope means; and

(e) the dimensions of said anodic aperture, the predetermined pressure of selected gas passed through said gas-inlet means and the predetermined electrical power adapted to be placed into said anode and said cathode bearing such relationship to one another that during operation of said source, the gas-operating pressure Within said envelope means is at least ten atmospheres and the total energy dissipated as plasma exiting through the anodic aperture provided through said envelope means is less than the total energy directed as radiations toward the radiation-transmitting envelope portion.

2. The radiation source as specified in claim 1, wherein the predetermined pressure of selected gas passed through said gas-inlet means and the size of the aperture provided through said anode and said envelope means bear such relationship to one another that when said source is operated with a power input of from 5 to 50 kilowatts, the gas flow through said aperture is from 2 to cubic feet per minute.

3. The radiation source as specified in claim 2, wherein the gas flow through said aperture is about 3.5 cubic feet per minute.

4. A radiation source comprising:

(a) envelope means of predetermined dimensions and configuration, a portion of said envelope means being radiation-transmitting, another portion of said envelope means formed as a radiation reflecting ellipse having only one focal point positioned within said envelope means, and the relative positioning of the radiation-reflecting envelope portion and the radiation-transmitting envelope portion being such that substantially all radiations generated proximate to said one focal point are ultimately directed toward the radiation-transmitting envelope portion and to the second focal point positioned exterior to said envelope means;

(b) a cooled anode and a cooled cathode operatively positioned within said envelope means and about said one focal point and adapted to be energized with a predetermined power input to sustain an intense arc discharge therebetween and proximate to said one focal point;

(c) an aperture of predetermined dimensions provided through said anode and extending through said envelope means and operable to conduct plasma therethrough;

(d) gas-source means terminating in gas-inlet means extending into said envelope means at a location which is axially displaced from said radiation-reflecting envelope portion and operable, during operation of said source, to pass a selected discharge-sustaining gas at a predetermined pressure into said envelope means to create a vortical gas stream moving over the radiation-reflecting portion of said envelope means and toward the anodic aperture provided through said envelope means; and

(e) the dimensions of said anodic aperture, the predetermined pressure of selected gas passed through said gas-inlet means and the predetermined electrical power adapted to be placed into said anode and said cathode bearing such relationship to one another that during operation of said source, the gas-operating pressure within said envelope means is at least ten atomspheres and the total energy dissipated as plasma exiting thorugh the anodic aperture provided through said envelope means is less than the total energy directed as radiations toward the radiationtransmitting envelope portion.

5. The radiation source as specified in claim 4, wherein said cathode is movable along the axis of said elliptical portion of said envelope means to insure that the hottest portion of the arc discharge adapted to be sustained between said anode and said cathode substantially coincides with said one focal point.

6. A radiation source comprising:

(a) envelope means of predetermined dimensions and configuration, a portion of said envelope means being radiation-transmitting;

(b) a cooled anode and acooled cathode operatively positioned within said envelope means and adapted to be energized with a predetermined power input to sustain therebetween an intense arc discharge of predetermined cross-sectional area, the surface of said anode facing said cathode having a generally flattened configuration which is oriented substantially perpendicular to said cathode and an area which is substantially greater than the cross-sectional area of the arc discharge adapted to be sustained between said anode and said cathode;

(c) an aperture of predetermined dimensions provided axially through said anode and extending through said envelope means, the cross-sectional area of such aperture as it opens into the flattened surface of said anode being substantially less than the cross-sectional area of the arc discharge adapted to be sustained between said anode and said cathode;

(d) gas-source means terminating in gas-inlet means extending into said envelope means at a location positioned about said radiation-transmitting envelope portion and operable, during operation of said source, to pass a selected discharge-sustaining gas at a predetermined pressure into said envelope means to create a stream moving toward the anodic aperture provided through said envelope means; and

(e) the dimensions of said anodic aperture, the pre- 7. A radiation source comprising: (a) envelope means of predetermined dimensions and configuration, a portion of said envelope means being radiation-transmitting, another portion of said envelope means being radiation-reflecting and having a focal point positioned within said envelope means, and the relative positioning of the radiation-reflecting envelope portion and the radiation-transmitting envelope portion being such that substantially all radiations generated proximate to said focal point are ultimately directed toward the radiation-transmitting envelope portion;

(b) a cooled anode and a cooled cathode operatively positioned within said envelope means and about said focal point and adapted to be energized with a predetermined power input to sustain therebetween an intense arc discharge of predetermined cross-sectional area, the surface of said anode facing said cathode having a generally flattened configuration which is oriented substantially perpendicular to said cathode and an area which is substantially greater than the cross-sectional area of the arc discharge adapted to be sustained between said anode and said cathode;

(c) an aperture of predetermined dimensions provided axially through said anode and extending through said envelope means and operable to conduct plasma therethrough, the cross-sectional area of such aperture as it opens into the flattened surface of said anode being substantially less than the cross-sectional area of the arc discharge adapted to be sustained between said anode and said cathode;

(d) gas-source means terminating in gas-inlet means extending into said envelope means at a location which is axially displaced from said radiation-reflecting envelope portion and operable, during operation of said source, to pass a selected discharge-sustaining gas at a predetermined pressure into said envelope means to create a vertical stream moving over the radiation-reflecting portion of said envelope means and toward the anodic aperture provided through said envelope means; and

(e) the dimensions of said anodic aperture, the predetermined pressure of selected gas passed through said gas-inlet means and the predetermined electrical power adapted to be used as input to said anode and said cathode bearing such relationship to one another that during operation of said source, the gas-operating pressure within said envelope means is at least ten atmospheres and the total energy dissipated as plasma exiting through the anodic aperture provided through said envelope means is less than the total energy directed as radiations toward the radiationtransmitting envelope portion.

8. A radiation source comprising:

(a) envelope means of predetermined dimensions and configuration, a portion of said envelope means being radiation-transmitting, another portion of said envelope means formed as a radiation-reflecting ellipse having only one focal point positioned within said envelope means, and the relative positioning of the radiation-reflecting envelope portion and the radiation-transmitting envelope portion being such that substantially all radiations generated proximate to said one focal point are ultimately directed toward the radiation-transmitting envelope portion and to the second focal point positioned exterior to said envelope means;

(b) a cooled anode and a cooled cathode operatively positioned within said envelope means about said one focal point and adapted to be energized with a predetermined power input to sustain therebetween an intense arc discharge of predetermined cross-sectional area and proximate to said one focal point, the surface of said anode facing said cathode having a generally flattened configuration which is oriented substantially perpendicular to said cathode and an area which is substantially greater than the cross-sectional area of the arc discharge adapted to be sustained between said anode and said cathode;

(c) an aperture of predetermined dimensions provided axially through said anode and extending through said envelope means, the cross-sectional area of such aperture as it opens into the concave surface of said anode being substantially less than the cross-sectional area of the arc discharge adapted to be sustained between said anode and said cathode;

(d gas-source means terminating in a plurality of gasinlet aperture means opening into said envelope means at a location positioned about said radiationtransmitting envelope portion and operable, during operation of said source, to pass a selected dischargesustaining gas at a predetermined pressure into said envelope means to create a vortical stream moving over the radiation-reflecting portion of said envelope means and toward the anodic aperture provided through said envelope means; and

(e) the dimensions of said anodic aperture, the predetermined pressure of selected gas passed through said gas-inlet means and the predetermined electrical power adapted to be used as input to said anode and said cathode hearing such relationship to one another that during operation of said source, the gasoperating pressure Within said envelope means is at least ten atmospheres and the total energy dissipated as plasma exiting through the anodic aperture provided through said envelope means is less than the total energy directed as radiations toward the radiation-transmitting envelope portion.

DAVID J. GALVIN, Primary Examiner.

GEORGE WEST-BY, Examiner.

D. E. SRAGOW, Assistant Examiner. 

1. A RADIATION SOURCE COMPRISING: (A) ENVELOPE MEANS OF PREDETERMINED DIMENSIONS AND CONFIGURATION, A PORTION OF SAID ENVELOPE MEANS BEING RADIATION TRANSMITTING, ANOTHER PORTION OF SAID ENVELOPE MEANS BEING RADIATION REFLECTING AND HAVING A FOCAL POINT POSITIONED WITHIN SAID ENVELOPE MEANS, AND THE RELATIVE POSITIONING OF THE RADIATIONREFLECTING ENVELOPE PORTION AND THE RADIATION-TRANSMITTING ENVELOPE PORTION BEING SUCH THAT SUBSTANTIALLY ALL RADIATIONS GENERATED PROXIMATE TO SAID FOCAL POINT ARE ULTIMATELY DIRECTED TOWARD THE RADIATIONTRANSMITTING ENVELOPE PORTION; (B) A COOLED ANODE AND A COOLED CATHODE OPERATIVELY POSITIONED WITHIN SAID ENVELOPE MEANS AND ABOUT SAID FOCAL POINT AND ADAPTED TO BE ENERGIZED WITH A PREDETERMINED POWER INPUT TO SUSTAIN AN INTENSE ARC DISCHARGE THEREBETWEEN AND PROXIMATE TO SAID FOCAL POINT; (C) AN APERTURE OF PREDETERMINED DIMENSION PROVIDED THROUGH SAID ANODE AND EXTENDING THROUGH SAID ENVELOPE MEANS AND OPERABLE TO CONDUCT PLASMA THERETHROUGH; (D) GAS-SOURCE MEANS TERMINATING IN GAS-INLET MEANS EXTENDING INTO SAID ENVELOPE MEANS AT A LOCATION WHICH IS POSITIONED ABOUT SAID RADIATION-TRANSMITTING ENVELOPE PORTION AND OPERABLE, DURING OPERATION OF SAID SOURCE, TO PASS A SELECTED DISCHARGE-SUSTAINING GAS AT A PREDETERMINED PRESSURE INTO SAID ENVELOPE MEANS TO CREATE A GAS STREAM MOVING TOWARD THE ANODIC APERTURE PROVIDED THROUGH SAID ENVELOPE MEANS; AND (E) THE DIMENSIONS OF SAID ANODIC APERTURE, THE PREDETERMINED PRESSURE OF SELECTED GAS PASSED THROUGH SAID GAS-INLET MEANS AND THE PREDETERMINED ELECTRICAL POWER ADAPTED TO BE PLACES INTO SAID ANODE AND SAID CATHODE BEARING SUCH RELATIONSHIP TO ONE ANOTHER THAT DURING OPERATION OF SAID SOURCE, THE GAS-OPERATING PRESSURE WITHIN SAID ENVELOPE MEANS IN AT LEAST TEN ATMOSPHERES AND THE TOTAL ENERGY DISSIPATED AS PLASMA EXITING THROUGH THE ANODIC APERTURE PROVIDED THROUGH SAID ENVELOPE MEANS IS LESS THAN THE TOTAL ENERGY DIRECTED AS RADIATIONS TOWARD THE RADIATION-TRANSMITTING ENVELOPE PORTION. 